1. Field of the Invention
This invention relates to the field of optoelectronics, and in particular to a method of stabilizing porous silicon structures suitable for use in photoluminescent and electroluminescent applications.
2. Description of Related Art
Porous silicon (PSi) formed by chemical or electrochemical etching of crystalline silicon in HF-based solutions is of considerable interest in the optoelectronics field because of its ability to produce bright photoluminescence (PL) at room temperature. While the origin of the PL was uncertain, it is now believed that the PL results from the quantum confinement of carriers within the silicon nanocrystals composing the porous layer even though there are contributions from the surface species.
Due to the fabrication process used, freshly prepared PSi surfaces are covered with silicon-hydrogen bonds (Si—Hx). This termination offers good electronic properties to the surface. However, the Si—Hx bonds are prone to hydrolysis when exposed to ambient air. A slow oxidation of the surface takes place and leads to the formation of surface defects, which are responsible for PL quenching and degradation of electronic properties of the material.
In any practical use of PSi layers for building optical devices, high PL and electroluminescence (EL) yields are required (external quantum efficiency (EQE)>1%). Typically, luminescent devices made from PSi are not stable and degrade with time due to oxidation of silicon-hydrogen bonds present on the surface. The luminescent intensity and electronic conduction properties diminish with time. There is therefore a need to stabilize such devices to prevent degradation of their properties over time. This can be achieved by passivation of the surface.
Thermal oxidation of the PSi surface is one of the most widely studied reactions to achieve a high PL stability, but this method destroys the porous layer integrity. A. Bsiesy et al. Surf Sci. 254, 195 (1991) have found that post-anodization of freshly prepared PSi layers in KNO3 or H2SO4 followed by chemical dissolution in HF solutions can be used for thinning the PSi walls. They have also shown that partially oxidized porous layers exhibit a large increase in the PL and EL intensities. The electrochemical oxidation of PSi surfaces is a very convenient and cheap method and can easily be used for mass production. The rate of the oxidation can be readily controlled because the amount of the oxide formed on the surface is proportional to the exchanged charge.
Electrochemical anodization of the freshly prepared PSi surface is a method of passivation that retains the porous integrity of the layer. This approach has been successfiully used for building electroluminescent devices with a high external efficiency (>1%). The electrochemical reaction requires hole consumption. Upon anodic polarization, a supply of holes from the substrate allows the electrochemical oxidation to occur at both the PSi walls and the bottom of the porous layer. Oxidation of the bottom part of the porous layer, however, breaks the electrical contact with the substrate and causes the end of the oxidation reaction. During this process, only the Si—Si back-bonds are oxidized and the Si—H bonds are not affected. This reaction leads to a surface that contains oxidized regions and non-oxidized ones. Even though growing an oxide film on the PSi layer offers a good surface passivation, PL quenching still occurs over time.
Recently, much effort has been devoted towards PSi passivation using chemical derivatization of the freshly prepared surfaces by replacing silicon-hydrogen (Si—Hx) bonds with Si—C or Si—O—C bonds, under various conditions, see, for example, J. M. Buriak, J. Chem. Soc. Chem. Commun. 1051 (1999); R. Boukherroub et al. Chem. Mater. 13, 2002 (2001). The organic modified PSi surfaces have shown good stability in different aqueous solutions of HF and KOH.
Such thermally or anodically oxidized products do not, however, fully satisfy the needs of industry, including high stability, the ability to retain the porous integrity of the material (no chemical etching during the thermal treatment), a low concentration of surface defects, the preservation of the PSi PL and EL, the possibility of controlling the wetting properties of the material by varying the nature of the end group, the availability of a wide range of functional groups compatible with the Si—Hx bonds, the possibility of introducing several functional groups on the surface in one step by reacting the freshly prepared PSi surface with a mixture of organic molecules, and the spatial control of the distribution of molecules on the surface (patterning).